A pharmacodynamic and pharmacokinetic study to determine the clinical significance of drug interactions between azathioprine and sulfasalazine, Balsalazide or Pentasa in patients with inflammatory bowel disease. The specific aims of the study are to estimate the proportion of clinically important leukopenia, defined as a total white blood cell (WBC) count <3,000 X 10 9/L, when SAS 4 g/day, Balsalazide 6.75 g/day, or Pentasa 4 g/day is combined with maintenance of remission treatment with a stable dose of 6MP or AZA in patients with one of the inflammatory bowel diseases (IBD), either ulcerative colitis (UC) or Crohn's disease. These estimates (and 95% CIs) will be computed at weeks 2, 4, 6, and 8 after adding SAS, Balsalazide, or Pentasa to AZA or 6MP therapy. The secondary aim is to estimate the change (and 95% CI) in total WBC count for each treatment group at weeks 2, 4, 6, and 8 after adding SAS, Balsalazide, or Pentasa to AZA or 6MP therapy.